1. Field of the Invention
The present invention relates to a laser diode driver (LD-driver) to drive a semiconductor laser diode (hereafter denoted as LD).
2. Related Prior Art
The LD-driver drivers the LD by receiving a driving signal and applying a modulation current modulated by the received driving signal. A conventional LD-driver usually includes a modulation current source and a bias current source with a current switching circuit operated by the driving signal. Supplying the driving signal with a differential mode to the current switching circuit with a paired transistor to switch the current. from the current source, the optical output from the LD may be modulated in accordance with the driving signal.
Japanese Patent Application published as JP-S60-187075A has disclosed a driver circuit for the LD, where one of the inputs of the differential circuit provides a capacitor to suppress the relaxation oscillation of the LD. This driver relaxes the leading edge of the modulation current, thus, relaxes the degradation of the optical output waveform due to the relaxation oscillation.
The conventional driver mentioned above has applied field effect transistors (FETs) for the differential circuit. The voltage controlled device, such as FET, may effectively control the current flowing in the. device by the input voltage because of their small trans-conductance. On the other hand, the bipolar device, typically a bipolar transistor with a current controlled configuration, seems to be hard to precisely control, in other words to control in analogue, the current flowing in the device because of their large trans-conductance.
Therefore, the present invention is to provide a LD-driver that, even if the driver is primarily constituted by current controlled devices, may effectively improve the output waveform of the LD by controlling the waveform of the modulation current precisely.